Science

March 15, 2010

I burn wood to heat my home. Otherwise, I use electric heat when I’m away from home. How can I determine my carbon footprint using the wood versus using electricity?

--Dot in Central New York

The comparative size of your carbon footprint depends on the source of your electric power and the quality of your wood burner. If the electric power used to heat your home is generated from coal or other fossil fuels, then your footprint might be bigger than if you're burning wood in a new, efficient, EPA-approved firebox or stove. But if you've got an old, inefficient stove or a traditional fireplace, your footprint could be way bigger than with electric heat. Also remember that you’ve got a major environmental tradeoff here: Even if the new wood burner has a smaller carbon footprint, per unit of energy it still releases a lot more other pollutants into the air than a coal-burning power plant, and it releases them right where you and your neighbors breathe. Burning wood to heat a home emits 250 times as much global-warming methane as burning coal in a power plant.

Electric heating is not such a great choice either. This is because only around 35 percent of the energy in coal gets converted to electricity; the rest is lost as heat, friction, turning the dynamo, and so on.

To calculate the difference between your two sources, you need to think in terms of British thermal units (Btus) and kilowatt-hours (kWh). Really, it's not hard. Bear with me. Since a typical generator's efficiency is only 35 percent, producing one Btu worth of electricity requires about 3 times as many Btus worth of coal to get the same amount of heat from electricity that you’d get from direct combustion of the coal. Since the carbon emissions generated to produce a Btu from burning wood are only a bit less than burning coal, to get 1 Btu from your wood, the total carbon dioxide released from the stove is roughly half that of coal-sourced electric power, provided you had a modern 75 percent-efficient wood stove. But if you've got an ancient stove, or an old-fashioned fireplace, which can be less than 15 percent efficient, then your carbon footprint could zoom up two or three times higher than with electric heat.

Of course, it’s not as easy to regulate the heat in a wood burner as in an electric heater, so you could end up burning more wood than you really need, pushing your carbon footprint up.

January 27, 2010

Mr. Green gets lots of questions from Sierra’s readership of 1.2 million that don't make it into the magazine or onto this blog. But he often replies to individual inquiries anyway, just for fun, and in the process sometimes even stumbles into exchanges that involve scientific discovery. For example, information about saving energy by filling empty fridge space with containers of water may emerge from the following discussion. It starts with a question from Megan in Gibsonville, North Carolina:

My Kenmore side-by-side refrigerator and freezer broke a few months ago and we had it fixed, but since then, the freezer has been empty except for ice. I shop locally and seasonally and we eat mostly fresh vegetables. I never use the freezer. Can I safely turn the freezer off, or is there something about the way the unit cools that will mess up the refrigerator half if I do this? I can't find anything in the owner's manual or online about this.

After several exchanges with one of Kenmore's technicians, who requested details such as the model number, we got the answer: “The freezer definitely needs to be turned on for the fridge to cool. If you turn it down too much, it won't keep the food. The customer has to find the appropriate settings that meet his needs."

To which I added, "So it seems like it should work keeping it on the lowest setting, but to be absolutely sure that setting the freezer too low doesn't affect the fridge, I recommend putting a thermometer in the fridge. It sounds like you guys are tech-savvy enough to do that."

Now I know she's tech-savvy is because while Megan was waiting for an answer, she e-mailed: "In the meantime, we heard the old 'fill it with milk jugs filled with water' thing. So my husband, who’s a physics professor, is trying to test this. He attached a Kill-A-Watt [an electric-use monitor] to the back of the refrigerator and has made a Google spreadsheet showing the temperature changes each hour (!), then tracking the changes after each time we add a milk jug filled with water."

Ever intent on encouraging citizen science, I asked her to share the results of this experiment. The latest word from her: "Our milk-jug experiment is going well. I'll definitely send you the results as we compile them. My husband is a perfectionist so it'll be up to me to actually convince him the data is 'good enough' to ship out."

So watch this space for the results. Or experiment with your own electric-use monitor (available for about $20 to $30) and track how much power individual appliances, computers, and other gadgets are using. You might be surprised at the waste caused by inefficient appliances, "vampire" energy drains from items left plugged in, and assorted stuff that doesn't really even need to be used.

October 24, 2008

Hey Mr. Green, I've heard that humans account for only 3 percent of global carbon emissions, with the rest coming from volcanoes, forest fires, and plant decay. Is this true, and if so, how can reducing our carbon footprint make an appreciable difference to the global situation? –Cary in Atlanta, Georgia

Hey Cary, Three percent may not sound like much, but if that small percent isn't reabsorbed each year, it keeps accumulating until you have a much bigger percent and a real problem. Think of it this way: If you only gain 3 percent in weight a year, it's not much. But if you never shed that weight, you'll balloon from a 135-pound lightweight-boxer to a full-size heavyweight (albeit a rather flabby one) in 20 years.

The natural world is estimated to emit a grand total of 770.3 billion tons of CO2 annually. Millions of life forms on land, from bacteria to elephants, pump out 440 billion tons of CO2, while the oceans release another 330 billion tons, and volcanoes add 300 million each year. Human activity, primarily from burning fossil fuel, but also from agriculture and forest burning and clearing, accounts for "only" 32.3 billion tons, New Scientist reports.

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